There’s a simple idea that two of West Germany's top car manufacturers seriously studying at the moment, both out of self interest ,but also out of concern for the environment. The concept is to develop vehicles that can run on a virtually limitless element hydrogen which when burned does not produce damaging fumes, but instead a bit of water vapour. The concept can solve two problems at once. First, it is a hedge for that day in the 21st century when hydrocarbon fuels run out, a prospect of no minor concern for the automotive industry. Beyond that, the increasingly dire warnings by environmental scientists about the 'greenhouse effect' in atomosphere caused by carbon dioxide exhausts adds urgency to the quest for a fuel that is less damaging to the environment. Of course, there is a hitch to hydrogen, both carmakers admit :though the know -how to run vehicles on nature’ slightest element is already available, hydrogen is far from being cost competitive compared to hydrocarbon fuels, and further refinements hydrogen -propulsion technology will be required. But what we are discussing today is the technology of the year 2020. But after several year's research Daimler and BMW engineers, in collaboration with other companies and research institutes in West Germany, independently have been tackling the technological and cost feasibility problems to be overcome in hydrogen fuel application. In addition to the two concerns of technology and economic feasibility, the carmakers say, there is the issue of safety. The spectacular explosion of the dirigible. Hindenburg in 1937 immediately comes to mind, and skeptics wonder what the German autobahn would look like in one of the hundred - car pileups that routinely happen every winter if all the cars and tanks loaded with hydrogen. A BMW engineer, Friedich Fickel, says that hydrogen is seen as less risky than gasoline. When leaked, hydrogen rises quickly up to the atmosphere, reducing the potential of explosion, whereas gasoline fumes linger close to the ground before dispersing. Still, both Daimler and BMW report that a considerable part of their development efforts are aimed at safe, lead - free storage of hydrogen fuel. The related question is what is the best method of storage. By now ,the tests by both carmakers have all but eliminated using hydrogen in gas form. As a gas it takes up about 14 times the space of liquid hydrogen and as much as 30 percent can be lost by leakege unless the tanks are perfectly sealed. Two other storage methods hold more promise. One is in liquid form, and the other in the form. of metal hydrides. In the latter, hydrogen if mixed with a metal alloy ,a process whereby the gas molecules are stored within the metal's molecular structure. Which of the following is the characteristic of hydrogen?